A strongly acidic cation-exchange resin, which is combined with a strongly basic anion-exchange resin to form a mixed bed and used in a condensate demineralizer of a fossil-fueled electric power plant or a PWR-type nuclear power plant, is usually a cation-exchange resin containing sulfonic acid groups as exchange groups, and the main constituent of substances eluted from the resin is polystyrenesulfonic acid (hereinafter, also referred to as “PSS”). It is known that, in a case where a strongly acidic cation-exchange resin has been degraded by oxygen, PSS is eluted, the eluted PSS fouls the anion-exchange resin, and the demineralization performance of the anion-exchange resin deteriorates. Moreover, when the degradation of the demineralization performance progresses, impurity substance ions leak at an exit of the demineralizer, and a degree of purity necessary for treated water cannot be ensured.
Therefore, it becomes necessary to adequately evaluate an ongoing performance of such a cation-exchange resin used in a condensate demineralizer and always to use a resin which has not degraded to an unacceptable degree. As a common method for evaluating the performance of a cation-exchange resin is to determine the PSS elution tendency. In practice, a method is employed wherein a strongly acidic cation-exchange resin is dipped or agitated in an aqueous extracting solution and after a predetermined time an amount of PSS eluted into the aqueous extracting solution is determined (for example, JP-A-9-210977). In this method, an amount of eluted PSS having a molecular weight of 10,000 or more, for example, is employed as an index of deterioration.
However, from recent investigation results, it has been recognized that, depending upon the nature of the structure of the matrix of a strongly acidic cation-exchange resin and the circumstance under which it is used, there is a case where a proper evaluation cannot be achieved by the conventional method for merely determining the amount of PSS elution. For even if the same category of strongly acidic cation-exchange resins are used, for example, there is a difference in distribution of molecular weight of eluted PSS between a gel-type strongly acidic cation-exchange resin and a porous-type strongly acidic cation-exchange resin, and when the distribution of molecular weight of eluted PSS is different between these resins, the degree of influence upon the PSS reaction to a strongly basic anion-exchange resin varies too.